As heavy everyday user of VoiceOver on several gadgets, I have maxed out the speech rate everywhere. Reason is simple - Many of the phrases of VoiceOver are repeted often and I sometimes already know what it is saying. And the rule is, the faster it is, the quiecker you can obtain the information.
As there is a terminal command on OSX available to speed up the fastest speech rate it is not possible on iOs, on mac it decreases back to Apple's limit the moment you change a speech language. As I often work in 2, sometimes 3 language environment, it made me stop that command. However, as I am very familiar with synthesised speech already, I would like to see it go faster.
So I suggest you to join in and send a message to "accessibility@apple dot com" to kindly ask them to consider this as an option. OF course, they may not do it, but one can never be shure if one hasn't done anything for it.
I hope you will join in and together we can make the usage of OSX and iOs even better.